Most of the documents in use today, for example, bank notes, credit cards, identity cards, railroad and airline tickets, checks and the like can be forged by means of modern reproduction processes at not too great an expense. Many proposals have been made, the object of which is the recording of authenticating features on such documents, which increase the cost of an attempted forgery and therefore reduce the probability of any forgery. A low probability of forgery is attained, if authenticating features are incorporated in the document in the form of optical microstructures, which diffract light in a characteristic manner. Such microstructures, such as, for example, holographically generated structures, phase diffraction gratings and kinoforms can only be manufactured at a high technical expense.
In documents having a substrate of thermoplastic material, the microstructures are embossed directly onto the substrate, as disclosed, for example, in British Patent No. 1,502,460. Paper documents are coated with a thin thermoplastic layer prior to the embossment of the documents with microstructures, as disclosed, for example, by Greenaway in U.S. Pat. No. 4,184,700. The microstructure can be protected from mechanical damage by means of a protecting layer in the form of a laminated foil, as disclosed, for example, by Greenaway, in U.S. Pat. No. 4,119,361, or by means of a lacquer layer, as disclosed for example, in G.B. Patent Application No. 2,082,593. The protective layer may also be impenetratable for visible light to hide the microstructure from view, as disclosed, for example, by Greenaway in U.S. Pat. No. 4,119,361; an opaque or dark protective layer of this kind is not, however, desirable in many cases, on one hand, for aesthetic reasons, and secondly, because it may provide a strong hint that the document carries therewithin a hidden authenticating feature. From Greenaway, U.S. Pat. No. 4,184,700, there is also known a document in which the protective layer has a different diffraction index than the thermoplastic embossment layer.
When subjecting such documents to a test to determine their authenticity, the microstructure is illuminated by a directional light ray, which has a predetermined wavelength, or a predetermined narrow wavelength region. One or several components of the light rays diffracted by the microstructure are sensed by a light sensing arrrangement, and an electronic evaluator or processor tests whether the intensity or the intensity ratio of these components lies or lie within expected limits, as taught, for example, by Greenaway, in U.S. Pat. No. 4,129,382. From Nyfeler, et al., U.S. Pat. No. 4,211,918, it is also known to embed a certain sequence of microstructures diffraction from one another in a document, and to test during authenticity examination, whether the sequence stored in the document coincides with a desired sequence stored in a read device.